Navigation and location-based services receivers which are configured to receive navigation service signals such as those from the GPLS, GLONASS, Galileo or Compass systems can benefit from the assistance of signals of opportunity, in which information derived from the signal of opportunity is used for example for location aiding, frequency aiding and time aiding by the location based service receiver. Examples of signals of opportunity which may be used by a location-based service receiver include GSM850, GSM900, GSM1800 and GSM1900, which use the standardised 850 MHz, 900 MHz, 1800 MHz and 1900 MHz frequency bands respectively, and WiFi, which uses the standardised 2.4 GHz frequency band.
Receiving signals of opportunity is facilitated by multi-band radio receivers, that is to say radio receivers that are capable of receiving signals in two or more frequency ranges. Multi-band radio receivers often require filters to reduce the effects of strong out-of-band blocking signals on the performance of the receiver. Such signals may be received, for example, from transmitters in close proximity to the receiver (e.g. a transmitter of a mobile telephone, if the receiver is used in a mobile telephone).
Typically, SAW (surface acoustic wave) filters are used to filter out such blocking signals. A typical receiver architecture having three receive paths, only one of which is operational at any one time, is illustrated schematically in FIG. 1. In the architecture 10 of FIG. 1, a multi-resonant antenna 12 is able to receive signals in three different frequency ranges. The output of the antenna 12 can be connected, via a switch or triplexer 13 to one of three parallel SAW filters 14, 16, 18, which are external to an integrated circuit (chip) in which parallel low noise amplifiers (LNAs) 20, 22, 24 are implemented.
Each of the LNAs 20, 22, 24 is configured to operate in one of the frequency ranges of the antenna 12, and receives at its input the output of a respective one of the SAW filters 14, 16, 18, which are configured to pass signals in the frequency range of interest and to attenuate out-of-band signals strongly.
The outputs of the LNAs 20, 22, 24 are fed to an input of a common buffer 26, via a common output resonator or tank circuit made up of a variable capacitor 28 and an inductor 20 connected in parallel between a supply voltage and the input of the buffer 26. The common output tank circuit can be tuned using the variable capacitor 28 to select its centre frequency.
A problem with architectures such as the one illustrated in FIG. 1 is that the bill of materials (BOM) cost can be high, as it is dominated by components external to the chip, and in particular by the SAW filters 14, 16, 18. The SAW filters 14, 16, 18 can, however, protect the LNAs 20, 22, 24 against extremely large co-located transmit signals because they have very good rejection of nearby frequencies in case the transmit and receive bands are separated in frequency. In such a multi-band receiver, it may be acceptable to receive on a different band at well-separated frequency when transmit is present on a first band, or simply to wait until the transmit signal is no longer present. Thus, it would be desirable to provide a receiver architecture which provides attenuation of out-of-band blocking signals without using costly SAW filters, yet still provide protection against damaging transmit signals.